This invention relates generally to cathode ray tubes (CRTs) and is particularly directed to color CRTs having a tensioned foil shadow mask attached to a flat glass faceplate.
CRTs having a flat faceplate with a flat tensioned foil shadow mask offer various advantages over conventional color CRTs of the curved shadow mask and faceplate type including greater picture brightness. The tensioned foil shadow mask is maintained in a stretched condition by securely attaching it to a generally rectangular mounting structure which is typically affixed to the inner surface of the flat glass faceplate. Maintaining the shadow mask in a tightly stretched condition ensures proper alignment of shadow mask apertures with phosphor elements disposed on the faceplate's inner surface. The phosphor elements are typically deposited on the faceplate by pouring a phosphor slurry on the faceplate's inner surface which is covered by a template to ensure proper positioning of the discrete phosphor elements thereon for each of the three primary colors. Following baking of the phosphor coated faceplate, excess phosphor is removed such as by washing, and CRT assembly continues.
The shadow mask mounting structure is typically in the form of four rails joined at respective adjacent ends thereof by corner brackets to form a generally rectangular support structure. The rails and brackets are generally metal and are attached by spot welding. A frit-based cement is used to affix the shadow mask mounting structure to the faceplate's inner surface. In the past, the shadow mask mounting structure has been a source of contamination as phosphor particles collect in the corners of the rail structure in gaps between the corner brackets and connected rails. These phosphor deposits remain after the excess phosphor is washed from the CRT front panel and later seep out from the gaps in the corner bracket-rail structure during subsequent CRT assembly. The phosphor particles which thus reappear represent a serious contaminant which frequently result in rejection or failure of the CRT. These contaminating phosphor particles may be deposited on the rail structure and prevent secure attachment of the tensioned foil shadow mask to its mounting structure. The loose phosphor particles may also be deposited upon the shadow mask and become lodged in its apertures. In either case, video image quality will be impaired. These charged phosphor particles may also come in contact with an electron gun and short it out. Attempts to remove phosphor particles by scrubbing the corners of the shadow mask mounting structure at various stages during CRT assembly have been labor intensive and thus costly, and have met with only limited success.
This invention addresses the aforementioned limitations of the prior art in the manufacture of color CRTs having flat glass faceplates and tensioned foil shadow masks by hermetically sealing the corner joints of the shadow mask mounting structure attached to the faceplate to prevent the collection of phosphor particles and their later discharge during subsequent CRT processing or operation.